Objective: Quantitative analysis of DNA in human metaphase chromosomes. Methodology: Quantitative cytochemistry, digital scanning cytophotometry, real-time interactive image processing and analysis by computer, biostatistical evaluation of results. These technologies are combined into a system of unique power and sensitivity that can detect and measure effects involving as little as 6x10-15g of DNA. Applications: a) establish the normal human karyotype in terms of DNA stain content and DNA-based centromeric index; b) study the families of persons with DNA marker chromosomes; c) measure chromosomes in patients with chronic myelogenous leukemia; and d) measure chromosomes in propositi with Downs' syndrome and their families, involving translocation of different regions of chromosome 21.